The present invention relates to aqueous non-ionically hydrophilised binding-agent dispersions based on polyurethane ureas with terminal polyethylene-oxide groups, to the preparation thereof, and to the use thereof for producing coatings.
Coating agents that are based on polyurethanes play a major role by reason of their outstanding properties such as, for example, high scratch resistance and low-temperature flexibility. By reason of increasingly stringent ecological and other statutory regulations, particular importance is attached to the solvent-free, aqueous polyurethanes. Coatings that are based on polyurethane dispersions find use in varying applications such as, for example, textile coating, plastics lacquering and automobile lacquering. For certain applications—such as in electronics, for example—it is necessary that coatings exhibit a conductivity that is as low as possible, in order to guarantee high electrical insulation. At the same time, it is desirable that the aqueous coating agents are stable as regards coagulation within a wide temperature-range, in order to be able to apply them using the differing processes—such as, for example, spraying or blade coating—under conditions that are as variable as possible.
Stabilisation of the polyurethane particles in an aqueous phase can be effected by means of two mechanisms, either by charges on the surfaces resulting in the repulsion of the particles (ionic stabilisation), or by water-soluble or partly water-soluble oligomer chains or polymer chains that are adsorbed on the surface or covalently bonded preventing an agglomeration by virtue of van der Waals interactions. (non-ionic stabilisation). The ionic stabilisation of dispersions of such a type results in systems that are relatively insensitive to coagulation with respect to an increase in temperature but that display enhanced conductivity by virtue of the presence of ions. In the case of non-ionic stabilisation, the situation is reversed—i.e. these systems can be destabilised by temperatures higher than 60° C., as described in DE-A 26 51 506, for example. However, these systems present the disadvantage of comparatively low conductivity. In addition, it is sufficiently well-known to combine the two stabilisation mechanisms, in order to obtain ionically/non-ionically stabilised dispersions that are stable as regards coagulation within a wide temperature-range, these systems likewise exhibiting higher conductivity by reason of charges.
The non-ionic stabilisation is based on the incorporation of functional hydrophilic polyethylene oxides that are reactive with respect to isocyanate groups (abbreviated in the following as PEG), it being possible for the PEG to be linearly, laterally or terminally incorporated in the main chain of the polyurethane.
In U.S. Pat. No. 3,905,929 non-ionic polyurethane dispersions are described in which PEG chains have been attached laterally in the polyurethane dispersion. The resulting binding agents are distinguished by relatively good mechanical properties. But the PEG groups are consequently not terminally anchored, and by-products may arise during the process.
In EP-A 0 317 258 difunctional, non-ionic PEG-containing emulsifiers are disclosed which can be employed for synthesising a polyurethane dispersion. The PEG-containing hydrophilising agents described therein are likewise incorporated in bifunctionally lateral manner. Although the polyurethane dispersions produced in such a way are distinguished by relatively high thermal stability, the hydrophilising agents have to be generated in a further process step, constituting a disadvantage from the point of view of economy.
EP-A 0 792 900 describes polyurethane dispersions for use in glass-fibre sizing materials. These dispersions may have been ionically and/or non-ionically hydrophilised, the polyisocyanate consisting, in a proportion amounting to at least 50%, of 1-methyl-2,4-diisocyanatocyclohexane and/or 1-methyl-2,6-diisocyanatocyclohexane. In the dispersions mentioned therein, quantities amounting to ≦15 wt. % of a non-ionic terminal hydrophilising component are employed.
In U.S. Pat. No. 5,066,732 non-ionic polyurethane dispersions are described, a monofunctional polyethylene-glycol component being converted in intermediate manner with a diisocyanate, and the reaction product subsequently being caused to react with a dihydroxyamine so as to yield a hydrophilising component. Accordingly, the polyethylene oxide is likewise anchored laterally in the polyurethane.
However, the polyurethane dispersions known from the state of the art all display inadequate stability in storage, particularly at elevated temperatures, and therefore, in addition, have a tendency to coagulate prematurely.
The object of the present invention consequently consisted in the provision of new aqueous polyurethane dispersions that exhibit a coagulation temperature of at least 75° C. and consequently possess adequate stability in storage.
The object has been achieved in accordance with the claims. Polyurethane-urea dispersions are claimed therein that in the polymer backbone exhibit monofunctional PEG-containing components having a certain molar mass and in a certain weight ratio terminally on the polyurethane chain.